The major goal of this project is to develop an efficient and versatile gene delivery system using Rous sarcoma virus (RSV) for the genetic therapy of cystic fibrosis. The enzyme catalyzed viral integration makes retroviruses the most promising vector four long-term gene therapy. We have shown that RSV has a relaxed requirement for acquiring functional envelope proteins. Functional incorporation of influenza HA (or other respiratory trophic) envelope proteins into RSV makes it possible to create hybrid RSV inducing particles, bearing envelope proteins that will target airway cells. This will improve the efficiency an specificity of infection, enhance the safety of the gene delivery procedure and provide the capacity for repetitive gene transfer without provoking the host immune response. Our efforts will be focused on three areas; (a) Develop hybrid retroviruses that are targeted at human airway cells. We will optimize the infectivity of HA bearing RSV and test for additional envelope proteins that can mediate viral entry into airway epithelial cells. (b) Develop hybrid virus delivery systems. This will include construction of versatile RSV vectors to meet different requirements of CF gene therapy and establish high efficiency packaging cell lines for virion production. (c) Develop a practical in vivo gene transfer protocol. This will require optimizing the transduction efficiency of RSV hybrid viruses using marker gene expression and functional complementation studies in CF airway cells. This project will provide a novel approach to vector development for CF gene therapy that capitalizes on the unique properties of RSV as a delivery system.